1. Field of the Invention
The present invention relates to a blow molding apparatus varying pitch between two rows of holding plates for holding a preform or final molding.
2. Description of the Related Art
In the past, a so-called one-stage molding apparatus produces a hollow vessel by moving a preform through injection molding, blow molding and releasing steps while the preform is held by a lip mold at the lip portion thereof. If a number of hollow vessels are to be molded through one cycle, two rows of neck mold holding plates are used with two corresponding rows of blow molds, as described in Japanese Patent Publication No. Hei 1-18847 (or U.S. Pat. No. 4,321,029). FIG. 12 is a cross-sectional view of the prior art system comprising an injection molding station 210 for injection molding a preform 224 and a blow molding station 212 for stretch blow molding a final product 222. Two rows of holding plates 216 for holding a lip mold 214 are disposed spaced apart from each other with a pitch P. For such a pitch P, two sets of injection cavity molds 218 are disposed in the preform injection molding station 210 and two sets of blow molds 220 are arranged in the blow molding station 212.
In such an arrangement, however, the pitch P must be relatively large, leading to increase of the entire system size.
This is because the pitch P must be determined in consideration of the release in the blow molding station 212. More particularly, the pitch P is required to be larger than two times the thickness of an inner split blow mold half 220a plus the movement of the mold on releasing or the maximum diameter of a vessel 222 to be molded. The thickness of the inner split blow mold half 220a must be selected so that it will not flex under a predetermined internal pressure (blow pressure). Thus, the thickness of the inner split blow mold half 220a plus a holding plate (not shown) attached to the back thereof will be increased. This forces the pitch P to be increased.
Japanese Patent Laid-Open No. Sho 61-125833 discloses a one-stage molding apparatus comprising two rows of holding plates, the row pitch being variable. The row pitch is decreased before and after the injection molding station. In the other stations, the row pitch is then increased into a magnitude required in the blow molding station. Therefore, the entire molding apparatus could be small-sized only into a limited level.
In U.S. patent application Ser. No. 07/634,079 U.S. Pat. No. 5,169,654 or Europe Patent Application No. 90/25770.9, the inventors have proposed blow molding process and apparatus which utilize two rows of preform holding plates and which can provide a small-sized structure by changing the row pitch in a blow molding station while increasing the number of moldings through one molding cycle.